fwiw; I had unexplained anxiety, fatigue, poor sleep and cognitive decline as a post-COVID induced histamine intolerance with a few bonus attributes. High amounts of histamine would accumulate in my mast cells then get released in 3-5am, undermining sleep and also inhibiting serotonin.
It took me about four years to fully understand my condition. Hope you figure out what is bothering you. The body is incredibly complex.